Coating of small bore articles



Aug. 14, 1962 A. w. DOLAN COATING OF SMALL BORE ARTICLES Filed Dec. 29, 1960 N mild! \\\P w m N w 1 P c L illlllllll w m N m ARTHURW. DOLAN INV EN TOR.

AT TO NEY 3,049,482 COATENG OF SMALL BORE ARTICLES Arthur W. Dolan, Peabody, Mass, assiguor to Sylvam'a Electric Products inc, a corporation of Delaware Filed Dec. 29, 1960, Ser. No. 79,375 6 Claims. (Cl. 204 181) This invention relates to a method of coating wire coils and more particularly to electrophoretically coating insulating material on tightly wound wire coils having generally tubular shapes and relatively small inside diameters.

Electrophoretic methods of coating objects have largely displaced such methods as dipping, spraying or plating for many purposes, particularly in the electronics industry. In the manufacture of vacuum tubes by the industry, the

electrophoretic methods have been widely used because of their numerous advantages.

Previously, however, there have been limitations to the use of such methods due to the size and shape of the object to be coated.

When coating heater coils, that are used in the fabrication of the vacuum tube, it has been highly desirable to obtain uniform products on a mass production basis. Groups of these coils must be similar in size, shape and physical appearance and any insulating coating applied to each of the coils must be of substantially identical thickness and height.

Heater coils having a small inside or bore diameter are an essential element in most vacuum tubes. These coils are placed inside thin walled, nickel sleeves which are coated on the outside with carbonates of barium, calcium and strontium. After a coil is inserted in a coated sleeve, the assembly is heated in a vacuum to reduce the carbonates to their corresponding oxides. When properly activated, electrons will be emitted and the coated sleeve will act as a cathode. Connection of the coil to a power supply will furnish heat to the sleeve to activate the oxides.

In the preparation of the heater coil, a layer of refractory oxide insulating material, preferably alumina, is cataphoretically coated to a portion of the exterior surface of a tightly wound wire coil. The coating mus-t insulate the coil from the sleeve since, if the metal of the sleeve touches the metal'of the coil, leakage or short circuits will occur. The coating must be uniform from coil to coil and the thickness and length of insulation on each must be substantially identical. Each coil must have a sharp, predictable delineation between coated and uncoated portions.

It would appear that the provision for a refractory oxide coating of predictable and uniform height on each coil would be relatively easy to obtain, especially when electrophoretic methods are utilized, since they involve dipping the article in a coating suspension; Unfortunately, clipping has a definite disadvantage since many heater coils have generally tubular shapes and relatively small bore diameters, usually less than about 10 mils. It has been found that the well known phenomena of capillary action causes the liquid refractory oxide coating suspension to rise inside the coil, above the upper level of the suspension bath and uniformity in coating height is almost impossible to obtain. A groupof heater coils with coatings of varying heights is unsatisfactory in the mass production of vacuum tubes, since uncoated'portions of the coil are to be welded to lead-in wires of the tube. If residual coating is present on the ends of the coil sputtering will occur when the 'weld'is made and weld failure will increase.

Another difliculty encountered from capillary action in these coating methods is electrical insulation of the coil from a holding clamp. Electrophor etic coating depends upon an electrical circuit, and clamps, which hold the coil the insulated heater coil positioned in a cathode sleeve.

3,049,482 Patented Aug. 14, 1962 10 have been olfered in the past but none, however, have proved completely satisfactory. For example, it was proposed to make the heater coil extremely long and to compensate for the capillary action by adjusting the dipping height. This proposal is not satisfactory since uniform 5 and predictable delineation between the coated and um coated portions is not obtained. Slight variations in the density of the suspension, atmospheric conditions or small differences in the bore diameter of the coils will result in wide variations in the height of the insulation coat.

20 Another proposed method was to coat the entire coil,

fire it and then chip off the unwanted portion of the coating. This method is not satisfactory since the removal of the excess coating is rather difficult and expensive, and uniformity in the finished product is lacking. Further- 25 more, the clamps which serve as electrodes during coating will gradually become insulated from the coil due to incrustation of the coating media.

Further it was proposed to coat the coil with the mandrel still in place. When a coil of this type is pre 3O pared, a wire of suitable diameter is wound around a mandrel. According to this proposal, the coil and man'- drel assembly are coated together and sintered, and the mandrel then removed by dissolving it in acid. Such methods of coating prevent capillary action since there 35 is no internal bore and heater coils are produced which have uniform and predictable delineations between coated and uncoated portions. Difiiculties were presented, however, since it was necessary to remove the mandrel through chemical action. The dissolvingstep' necessitated intro- 40 ducing ions into the system which would contaminate the coating and cause leakage between the coil and the oath ode sleeve. I

The method of preparing the heater coil accordingto this invention utilizes in a treating operation the very principle that must be eliminated in the coating operation. It has been discovered that capillary action may be used to advantage if the proper steps are followed, since when a liquid rises in the bore of the coil capillary action tends to retain it therein. The liquid filling is rather difficult to dislodge and will remain inside the coil, even during subsequent dipping operations. Rather than flowing from the bore when the coil is removed from the liquid, it will be retained by capillary action. Thus, when the bore is filled with a predipping liquid, no additional capillary action will be evidenced in subsequent dipping operations. Upon immersing the predipped heater coils in a refractory oxide insulating suspension, uniform and predict-able delineations between coated and uncoated por tions are evidenced precisely at the upper levelof the refractory oxide suspension bath. Firing the'coil will remove the predipping liquid by vaporization.

FIGURE 1 of the drawing is a schematic view of an assembly line which may be used in preparing an insulated heater coil. i

FIGURE 2 is a cut-away section of a vacuum tube with FIGURE 3 is an elevational view of the coated heater coil before firing. The sharp delineation obtained between the coated and uncoated portions is clearly shown in this FIGURE 4 is an enlarged cross section of the coated heater coil taken along the lines 4,4 of FIGURE 3. This figure shows the coating of insulating material on the outside of the coil and the bore-filling liquid on the inside.

The various steps in the production of the insulated heater coil are shown in FIGURE 1, which is illustrative of the double indexing treating method now utilized. In such double indexing methods two coils are treated simultaneously; however, each coil is dipped but once in any given bath. After dipping, a unit of two clamps will move laterally down the production line eventually to a position over an adjacent bath. If double dipping in a bath is desired, however, provision may be made by adjusting the motion of the clamps. The clamps 1 which carry the coils through the various steps in the process are of identical construction. Each clamp is adapted to be connected to, a source of electricity to provide for electrophoretic coating. A clam-p 1 comprises the two retaining jaws 2 and 3 of which jaw 3 is stationary and rigidly affixed to mount 4. A movable jaw 2 pivots about the shaft 5. A spring (not shown) is placed within the mount 4 to bias a spring retainer 6 which is attached to movable jaw 2, against stationary guide 3. Spring retainer 6 holds the heater coil 7 stationary during the various dipping steps of this process. Although the clamp shown is preferred, it is apparent that many other types of clamps may be adapted to this process with equal success. Uncoated heater coils 7, having a bore diameter less than about 10 mils, are dipped in the predipping bath 10 in positions A and B. After dipping in bath 10, the bore of the heater coil is filled with the bore-filling liquid, which will be retained therein during the subsequent dipping operations due to capillary action.

The selection of suitable liquids for predipping is quite important when an electrophoretic coating process is utilized. Primarily, it is necessary to select a liquid that exhibits sufficient capillary action in the bore of the coil to remain therein while it is transferred from the predip ste to the electrophoretic coating step. Furthermore, the liquid must not contaminate the insulation in the finished heater coil and it must be rather volatile so when the heater coil is fired to sinter the refractory oxide the liquid will be driven off. In addition to the above criteria, it is also essential that the predip liquid be compatible with the insulation suspension. When electrophoretic coating procedures are followed, it is important that the coating suspension contain only certain ingredients in certain critical proportions. An example of such materials is recited in the co-pending application of George Bouchard, Sahag R. Dakesian and Arthur W. Dolan entitled Electrophoretic Coating and Process, Serial No. 736,994, filed May 22, 1958, now Patent No. 2,966,449. If the proper materials are not maintained in these critical proportions the electrophoretic coating will not take place. Additionally the predip liquid must be compatible with the solvent system chosen for the electrophoretic coating. For these purposes it has been found that polyethylene glycol polymers are suitable and preferably polyethylene glycols having a molecular weight between 350 and 550. In order to prevent lumping, since the polymer is viscoidal, 30 to 70% by weight of a suitable solvent such as methanol may be added.

After the predipping, the coils are moved to a transfer position C and then to rinsing positions D and E. Tank contains a liquid suitable for washing the excess polyethylene glycol from the exterior surfaces of the coil, for example, methanol. This rinsing step, however, will not remove the bore-filling liquid of the predipping operation. As noted above, it is essential to maintain the proportions of ingredients in the electrophoretic coating, suspension. within critical limits and if the excess predipping liquid remained on the coil, these proportions might be disturbed.

After dipping in tank 20 the heater coils are moved to transfer position F and thence to positions G and H, wherein they are dipped in a refractory oxide coating suspension. This suspension is pumped into the system through the conduit 31 positioned at the bottom of tank 30; the shape of the tank 30 and the position of the ingress conduits causes the insulating material to pass upwardly as a diverging stream and flow into the open ends of conduits 32 and 33. Power sources indicated schematically as elements 36 and 37 are connected at one end to each of open ended conduits 32 and 33 and at the other end to each of the clamps 1. The open ended conduits serve as the anodes in the coating process and the clamp which holds the heater coil will be the cathode when the process is cataphoretic, but would be reversed if the process were anaphoretic. When electricity is applied, the refractory oxide is coated to the heater coil. After coating, the unused circulating refractory oxide suspension is removed through conduits 34 and 35.

The heater coils are moved from positions G and H to transfer position I and thence to dipping positions I and K wherein the excess refractory oxide coating applied in bath 30 is removed. A liquid that does not contaminate the coating and which may be easily removed in subsequent operations is used in this bath and it has been found that, for example, Z-nitropropane is suitable. After dipping in bath 40, the heater coils are moved to positions M and N and dipped in tank 50 to remove the excess 2- nitropropane. It is necessary that this bath contain a liquid which is quite volatile and easily removed and, furthermore, it must not contaminate the coating on the heater coil. Solvents such as petroleum ether may be used.

The heater coil is then transferred to position 0, removed from the clamp and afterward fired to sinter the refractory oxide coating and vaporize the bore filling liquid to produce a finished heater coil.

The coated heater coil is shown in FIGURES 3 and 4 of the drawing. Such coils are generally fabricated in a hairpin shape; however, other shapes may be utilized if desired. Both the coated portion 19 and the uncoated portions 12 and 14 of the coil have the same generally tubular shape. Each uncoated portion 12 and 14 of the coil will be welded to appropriate lead-in wires in a vacuum tube. The coated portion 19 of the coil will be slipped into the cathode sleeve.

The cross section in FIGURE 4 illustrates the filling liquid inside the coated, but yet unfired, heater coil. The liquid is retained during the coating operations and its removal takes place only during the firing step. The layer of insulating material 19 is shown on the exterior surface of the coil 18. To a limited degree, some of the insulation coating impregnates the winding.

The heater coil positioned in the cathode sleeve of a vacuum tube is shown in FIGURE 2. Each uncoated portion 12 and 14 of the hairpin shaped heater coil 7 is welded to lead-in terminals 24 and 25 of vacuum tube 23 and the coated portion 16 is slipped in the sleeve 22. When energized, the heater coil will transmit heat to coated cathode sleeve 22 which produces electron emission.

It is apparent from the foregoing that while this invention has particular applicability in the preparation of heater coils, it is equally applicable to many other procedures for coating relatively small bore articles where capillary action causes the coating media to rise in the bore of the coil. Therefore, although what has been shown constitutes the preferred embodiment of the present invention, various changes and modifications will suggest themselves to those skilled in the art. The invention, therefore, should not be limited to the exact details shown, but only by the spirit and scope of the appended claims.

As my invention I claim:

1. The process for coating an elongated tubular article which tends to exhibit capillary action upon being placed in a coating bath, the steps for coating said tubular article and eliminating said capillary action which comprise: placing said tubular article in a pre-dipping bath of a bore-filling liquid whereby the liquid will fill the bore of said tubular article and remain held therein by capillary action, removing a filledtubular article from said predipping bath and placing said filled tubular article in a coating bath, removing a coated, filled tubular coil from said coating bath.

2. The process for coating a tightly wound coil having a generally tubular shape which tends to exhibit capillary action upon being placed in a coating bath, the steps for coating said coil and eliminating said capillary action which comprise: placing said coil in a pre-dipping bath of a bore filling liquid whereby the liquid will fill the bore of said coil and remain held therein by capillary action, removing a filled coil from said predipping bath and partially immersing said filled coil in a suspension of coating material, removing a coated, filled coil from said suspension.

3. The process for coating a tightly wound wire coil having an internal bore diameter less than about mils, the steps for eliminating capillary action and coating said coil which comprise: placing said coil in a predipping bath of a bore filling liquid whereby the liquid Will fill the bore of said coil and remain held therein by capillary action, removing a filled coil from said predipping bath and placing said filled coil in a bath of coating material, removing a coated, filled coil from said coating bath.

4. The process for cataphoretically coating a tightly wound wire coil having a generally tubular shape which tends to exhibit capillary action upon being placed in the cataphoretic coating bath, the steps for eliminating said capillary action and coating said coil which comprise: placing said wirre coil in a predipping bath of a bore filling liquid whereby the liquid will fill the bore of said coil and remain held therein by capillary action,

removing a filled coil from said predipping hath, partially immersing said filled coil in a suspension of coating mai terial and applying an electric current to coat the suspension on said coil, removing a coated, filled coil firom said coating bath.

5. The process for cataphoretically coating a tightly wound helical Wire coil having a generally tubular shape which tends to exhibit capillary action upon being placed in the coating bath, the steps for eliminating said capillary action and coating said coil which comprise: placing said coil in a predipping bath of polyethylene glycol thereby filling the bore, removing a filled coil from said predipping bath, placing said filled coil in a suspension of refractory oxide and applying an electric current to coat said suspension to said filled coil, removing a filled, coated coil from said refractory oxide suspension.

6. The process for coating a tightly WOllIld helical coil having an internal bore diameter less than about 10 mils, the steps for eliminating capillary action of coat-- References Cited in the file of this patent UNITED STATES PATENTS Veeder Aug. 11, 1959 Currah et al. Feb. 23, 1960 

1. THE PROCESS FOR COATING AN ELONGATED TUBULAR ARTICLE WHICH TENDS TO EXHIBIT CAPILLATY ACTION UPON BEING PLACED IN A COATING BATH, THE STEPS FOR COATING SAID TUBULAR ARTICLE AND ELIMINATING SAID CAPILLARY ACTION WHICH COMPRISE; PLACING SAID TUBULAR ARTICLE IN A PRE-DIPPING BATH OF A BORE-FILLING LIQUID WHEREBY THE LIQUID WILL FILL THE BORE OF SAID TUBULAR ARTICLE AND REMAIN HELD THEREIN BY CAPIL- 